Our initial studies on adhesion among chick embryo neural cells indicated that aggregation of single cells prepared from retina and brain tissue occurs by a common mechanism. Further investigation revealed that a cell surface polypeptide with a molecular weight of 140,000 was involved in cell adhesion. Specific antibodies against this molecule and Fab' fragments prepared from the antibodies were found to inhibit aggregation of both retinal and brain cells. Cells from 8-day embryos, which aggregate rapidly, were found to have 3 to 4 times more of the 140,000 MW polypeptide on their surfaces than cells from 14-day embryos, which aggregate at about one-forth the rate of 8-day cells. During the next year, our studies will focus on the use of the specific antibodies to localize the 140,000 MW molecule in tissues at different developmental ages, and to perturb its function during embryonic tissue formation. These studies should more clearly define the role of the 140,000 MW polypeptide in cell adhesion, and of cell adhesion in physiological mechanisms during ontogeny.